Once a gas or oil well bore has been drilled, casing is typically lowered into the well bore. The casing is then cemented into place by pumping a liquid cement slurry into the annular space between the casing and the well bore. This generally requires displacement of drilling fluid in the annulus by the cement slurry.
Once the cement slurry is in place, it must be permitted to harden and solidify before operations relating to drilling and completing the well can be resumed. Because the cemented annulus extends thousands of feet into the ground, it is difficult to know when the solidification of cement is complete. Due to the high cost of rig time, there is an incentive to accurately monitor the solidification process and, thus, minimize the delay in operations.
U.S. Pat. No. 5,377,753, issued to Haberman et al., discloses a technique of transmitting pressure waves down the well bore from the surface of the cement slurry, and measuring the time required for the waves to reflect back to the surface. The pressure waves can be transmitted using a fluid, such as air or water, which is injected at the surface. The cement generally becomes solid at the bottom of the well first, because of the higher temperature. The solidification then progresses up the well. The reflection of pressure waves from the highest location of set cement can thus be used to measure the progress of the setting.
U.S. Pat. No. 4,769,601, issued to Herrick, discloses a testing method which uses nuclear magnetic resonance to determine the setting time of cement. This method is not adapted for use in situ in an oil well.
There is a need or desire in the oil industry for an improved testing method for monitoring the setting of cement in an oil well bore.